The present invention relates to a color filter used for color liquid crystal display unit, and in particular to a color filter having special features in black matrix.
In a liquid crystal display unit, at least two transparent substrates made of the material such as glass having transparent electrodes are provided face to face to each other with spacings of 1 to 10 .mu. and liquid crystal substance is sealed therebetween and the liquid crystals are oriented in a certain direction by applying voltage across the electrodes so that transparent and non-transparent portions are formed to display the image. In a color liquid crystal display unit, three-color filters for red (R), green (G) and blue (B) corresponding to three primary colors of light are furnished on one of transparent electrode substrates. By adjusting the voltage applied on the transparent electrode, the transmission of liquid crystal light is controlled. Thus, by controlling the light quantity of the light transmitting 3 color filters for R, G and B, color display is accomplished by developing colors through the addition of 3 primary colors.
The layers colored with the colors of R, G and B of a color filter are formed on a substrate where black matrix is provided for defining the boundary of the colors. Black matrix defines three primary colors of R, G and B of the color layers, and it also shields the light for the electrodes for driving of liquid crystal or transistor such as TFT on the substrate placed face-to-face to the color filter.
The black matrix formed on the color filter is usually formed by thin film of metals such as chromium, nickel, aluminum, etc.
To form the black matrix, thin film of these metals is placed on glass substrate by the methods such as sputtering, vacuum evaporation, etc. Then, photo resist patterns are formed by the technique of photolithography, and the pattern is formed by etching metal thin film using photo resist pattern as the etching mask.
Although black matrix made of metal thin film is formed by the methods such as sputtering, vacuum evaporation, etc., the cost for forming film is expensive. Because metal thin film has metallic luster, the reflected light from metal chromium thin film is increased in case the light entering the display unit from outside is strong, and this results in the deterioration of display quality in a transmission type display unit such as liquid crystal display unit.
For the purpose of decreasing the reflectivity of metal chromium, it is proposed to form chromium thin film after a film of low reflectivity such as chromium oxide is placed on glass substrate before metal chromium thin film is formed. However, this is not very desirable because more processes are required and the manufacturing cost is increased.
Under such circumstances, the present inventor has proposed the following manufacturing method in the Japanese Patent Application which describes a color filter having black matrix made of polyimide type resin with dispersed light shielding material such as carbon black as the black matrix with low reflectivity and low manufacturing cost, and which also discloses a method for manufacturing such color filter.
Description is now given on the manufacturing method of black matrix of polyimide type in connection with FIGS. 2 (A)-2 (F), which show a cross-sectional views of color filter: To a glass substrate 10, which has been washed well by conventional method as shown in FIG. 2 (A), pre-treatment for increasing the bonding strength with polyimide to be formed is performed using a pre-treatment agent 11.
On the glass substrate thus pre-treated, carbon black, metal oxides such as triiron tetraoxide or mixture of these substances are dispersed in polyamic acid solution, and this is coated on the glass substrate 10 by roll coating method or spin coating method to form the coated layer 12 as shown in FIG. 2 (B).
After polyamic acid is coated, pre-baking is performed as shown in FIG. 2 (C). Then, photo resist is coated on the coated layer of polyamic acid compositions to form a photo resist layer 13. The photo resist layer is exposed to light using a predetermined photo mask 14 as shown in FIG. 2 (D) and the image is developed. Thus, the desired pattern is formed on the photo resist layer.
Then, as shown in FIG. 2 (E), the coated layer of polyamic acid is etched according to the pattern of the photo resist layer, and the photo resist layer is detached as given in FIG. 2 (F).
When the photo resist layer on the black matrix 15 is detached through oxidation as in the manufacturing process by photolithography in a semiconductor manufacturing system, black matrix is also damaged as it is an organic substance located in lower layer of the photo resist layer. For this reason, it is practiced to selectively dissolve and detach only the photoresist layer using various types of organic solvents.
In case novolak type photo resist is used, the following various types of organic solvents can be used: Ketone type organic solvents such as acetone, methylethylketone, etc., alcoholic type organic solvents such as methanol, ethanol, etc. cellosolve type organic solvents such as ethyl cellosolve, cellosolve acetate type solvents such as ethyl cellosolve acetate, ester type organic solvents such as butyl acetate, or glycol type organic solvents.
For the treatment to increase the bonding strength between glass substrate and polyimide, aminosilane type coupling agent such as .gamma.-(2-aminoethyl)-aminopropyltrimethoxysilane, .gamma.-aminopropyltriethoxysilane, etc. having high affinity with silicon, which is a component of glass, has been used.
To treat the glass substrate by an aminosilane type coupling agent in order to increase bonding strength, it is necessary to use coupling agent for coating process and drying process.